Uphill energy transfer in photosystem I from Chlamydomonas reinhardtii. Time-resolved fluorescence measurements at 77 K

Wojciech Giera, Sebastian Szewczyk, Michael D. McConnell, Kevin Redding, Rienk van Grondelle, Krzysztof Gibasiewicz

Research output: Contribution to journalArticlepeer-review

11 Scopus citations


Energetic properties of chlorophylls in photosynthetic complexes are strongly modulated by their interaction with the protein matrix and by inter-pigment coupling. This spectral tuning is especially striking in photosystem I (PSI) complexes that contain low-energy chlorophylls emitting above 700 nm. Such low-energy chlorophylls have been observed in cyanobacterial PSI, algal and plant PSI–LHCI complexes, and individual light-harvesting complex I (LHCI) proteins. However, there has been no direct evidence of their presence in algal PSI core complexes lacking LHCI. In order to determine the lowest-energy states of chlorophylls and their dynamics in algal PSI antenna systems, we performed time-resolved fluorescence measurements at 77 K for PSI core and PSI–LHCI complexes isolated from the green alga Chlamydomonas reinhardtii. The pool of low-energy chlorophylls observed in PSI cores is generally smaller and less red-shifted than that observed in PSI–LHCI complexes. Excitation energy equilibration between bulk and low-energy chlorophylls in the PSI–LHCI complexes at 77 K leads to population of excited states that are less red-shifted (by ~ 12 nm) than at room temperature. On the other hand, analysis of the detection wavelength dependence of the effective trapping time of bulk excitations in the PSI core at 77 K provided evidence for an energy threshold at ~ 675 nm, above which trapping slows down. Based on these observations, we postulate that excitation energy transfer from bulk to low-energy chlorophylls and from bulk to reaction center chlorophylls are thermally activated uphill processes that likely occur via higher excitonic states of energy accepting chlorophylls.

Original languageEnglish (US)
Pages (from-to)321-335
Number of pages15
JournalPhotosynthesis research
Issue number2
StatePublished - Aug 1 2018


  • Chlamydomonas reinhardtii
  • Excitation energy transfer
  • Light-harvesting complex I
  • Photosystem I
  • Red chlorophylls
  • Time-resolved fluorescence

ASJC Scopus subject areas

  • Biochemistry
  • Plant Science
  • Cell Biology


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